Like the transceiver or the radio set for business use, a terminal having a function (PTT: Push-to-talk) of exchanging a voice alternately between a plurality of speakers in such a manner that the speaker can talk while pushing a talking button provided to the terminal and can listen the talking contents of other speaker output by a voice while releasing the talking button is widely used nowadays. Unlike the terminal utilizing the telephone line that allows basically the talking between two persons, the terminal utilizing this PTT function is often employed in a situation that the user has simultaneously a communication with plural persons since such terminal permits easily three persons or more to talk together.
It is the technology called PoC (Push-to-Talk over Cellular) that implements this PTT function by the cellular phone. The cellular phone utilizing the PoC function can be used to do a PoC talking by selecting a person the user talks to on the phone from a phone book and then pushing/releasing a particular button provided to the cellular phone like the transceiver or the radio set for business use, for example. Since the cellular phone utilizing the PoC function does not need a dialing process executed when the terminal utilizing the telephone line starts the talking, it can shorten a time required until the talking is started. Processes in the cellular phone utilizing the PoC function (referred simply to as the “cellular phone” hereinafter) and the server utilized in this PoC talking (referred simply to as the “server” hereinafter) will be explained concretely hereunder.
An example of a sequence in a system constructed by cellular phones and a server in the prior art is shown in FIG. 16. In FIG. 16, the situation that the PoC talking is done between a cellular phone A owned by a user A and a cellular phone B owned by a user B is supposed.
The most important scheme in the PoC talking process between the cellular phones and the server is that the server can manage the right that grants a particular cellular phone to talk (called the floor) and the cellular phone to which the floor is granted can do the talking. This scheme is explained with reference to FIG. 16 hereunder. First, when the user B executes the operations (e.g., selects the user A as the person on the other end from the phone book and pushes the particular button) to start the PoC talking with the user A by the cellular phone B, the cellular phone B requests the server to grant the floor (step S1601). Then, when the server is informed from the cellular phone B that the PoC talking is started between the cellular phone A and the cellular phone B, such server transmits a signal to grant the floor to the cellular phone B (step S1602). Then, the server stores a status of the floor indicating the “Floor Grant”. When the cellular phone B gets the floor from the server, the user B can start the talking and can continue to talk while the particular button is pushed (a period of time encircled by a dotted line after step S1602).
In contrast, when the cellular phone A that received the start of the PoC talking from the cellular phone B accepts the operation (pushing the particular button) from the user A to talk with the user B, such cellular phone A requests the server to grant the floor (steps S1603, S1605). At this time, when the server is requested from the cellular phone A to grant the floor during a period of time a status of the floor is the “Floor Grant”, such server transmits a signal informing that “the floor cannot be granted” to the cellular phone A (steps S1604, S1606). As a result, even when the user A pushes the particular button, such user A cannot talk while the cellular phone A fails to get the floor from the server (a period of time the floor is granted to the cellular phone B). Then, the cellular phone B releases the floor (step S1607), and then the cellular phone A requests the server to grant the floor (step S1608). Then, the server grants the floor to the cellular phone A (step S1609).
Also, another example of the sequence in the system constructed by cellular phones and a server in the prior art is shown in FIG. 17. In FIG. 17, the situation that the PoC talking is done between cellular phones A, B, C owned by three persons of users A, B, C respectively is supposed.
First, the user B executes the operation (e.g., selects the users A, C as the person on the other end from the phone book and pushes the particular button) by the cellular phone B to start the PoC talking with the users A, C. The subsequent processes of the cellular phone B and the server required until the user B starts the talking are identical to the processes of the cellular phone B and the server in the explanation in FIG. 16.
When the user B executes the operation (releases the particular button) by the cellular phone B to end the PoC talking after the user B ended the talking, the cellular phone B informs the server that the floor is released (step S1701). When the server is informed from the cellular phone B that the floor is released, such server updates the “Floor Grant” to a status of the floor indicating the “Floor Idle”.
Then, the user C executes the operation (push the particular button) by the cellular phone C to start the PoC talking with the users A, B, and the cellular phone C requests the server to grant the floor (step S1702). Then, since a current status of the floor is the “Floor Idle”, the server transmits a signal to grant the floor to the cellular phone C (step S1703). Then, the server updates a status of the floor to the “Floor Grant”.
Subsequently, the server repeats three processes given as following.
(1) When the server is requested by another cellular phone to grant the floor during a period of time a status of the floor is the “Floor Grant”, such server transmits the signal to another cellular phone to inform that the floor cannot be granted.
(2) When the server is informed from a particular cellular phone that such phone releases the floor (steps S1701, S1704, S1707), such server updates the “Floor Grant” to a status of the floor indicating the “Floor Idle”.
(3) When the server is requested by the cellular phone to grant the floor during a period of time a status of the floor is the “Floor Idle” (steps S1702, S1705), such server transmits the signal to grant the floor to the cellular phone (steps S1703, S1706). Then, the server updates a status of the floor to the “Floor Grant”.
Non Patent Literature 1: Push-to-Talk over Cellular (PoC)-Archtecture-2.0.8, page 6